In leakage detectors termed counterflow leakage detectors since the test gas enters into a high vacuum pump system (one or several series connected high vacuum pumping stage(s), preferably friction pumping stages) through an intermediate inlet, and flows upstream against the pumping action of the pump system towards the test gas detector. Leakage detection is performed such that the inlet of the instrument is connected to a unit under test, for example, being sprayed from the outside with test gas. In the instance of a leakage being present, the test gas penetrates into the unit under test and is detected with the aid of the test gas detector. A test chamber may also be connected to the inlet of the leakage detector in which one or several test gas containing units under test are accommodated. During the leakage detection process and in the instance of one or of several objects under test being leaky, test gas enters into the test chamber and is detected with the aid of the test gas detector. The vacuum leakage detection process detailed, may be performed qualitatively or quantitatively. As the test gas, helium has been found well-proven.
From U.S. Pat. No. 4,472,962 a leakage detector of the here affected kind is known. A test chamber is connected to its inlet. Evacuation of the test chamber is performed through the fore-vacuum pump which is necessary for operating the high vacuum pump system being implemented by way of a turbomolecular vacuum pump. During the leakage detection process, the gas which is to be analysed for the presence of the test gas is supplied to an intermediate inlet at the turbomolecular pump.